1. Field of the Invention
The present invention relates to a shearing method for a thin plate in which a product part such as a spacer composed of a thin plate, for example, with the thickness of not more than approximately 0.3 mm is formed by shearing.
2. Description of the Related Art
When various types of devices are assembled or some components are joined, the height, width or length with a high degree of accuracy is sometimes required. However, individual component has a variation due to material itself and machining and thus, when the individual part is assembled or joined, a prescribed accuracy is not attained due to the accumulated error. Therefore, as shown in FIG. 7, in order to correct the accumulated error, adjustment work is performed such that a spacer 102 composed of a thin plate is interposed between parts 100 and 101 to obtain a specified dimension. The accumulated error is individually different according to a product but, in normal cases, it is in the range of about 0.05 mm–0.5 mm. Accordingly, plural kinds of spacers each of whose thickness is respectively different, for example, by 0.05 mm within the range of the accumulated error, are provided beforehand and an appropriate spacer is used according to the error.
One of the most inexpensive manufacturing methods for such spacer 102 is a punching work by means of a press. When a punching work is performed on a thin plate, a projection such as a burr 103 is formed at a peripheral edge on one side face of the thin plate. Therefore, when the spacer 102 having the burr 103 is interposed between the parts 100 and 101, a gap G is formed between the spacer 102 and the part 100 by the burr 103, which causes to be unable to adjust. Accordingly, the punching work is not adopted.
The spacer 102 is commonly produced by etching processing in a prescribed shape in order to avoid forming of the burr 103. However, the etching processing requires a long producing time and thus increases the cost for producing the spacer. Since it is necessary for the spacer 102 to be provided with plural kinds of plate thickness beforehand, the required number of spacers is in the range from several times to several tens of times more than the number of the product. Further, many spacers may not be used, which causes to increase the cost higher.
A method for making the burr small as much as possible in punching work for a thin plate by a press has been proposed. A punching method for a thin plate will be described below with reference to FIG. 8, which is disclosed in Japanese Patent Laid-Open No. Hei 7-195130 and in Japanese Patent Laid-Open No. 2001-284116.
FIG. 8(A) is a sectional side view showing a state before a plate 100 is sheared. The plate 100 is held by an upper die 110 and a lower die 111. The upper die 110 and the lower die 111 are respectively provided with a hole 114 into which the upper punch 112 is inserted and a hole 115 into which the lower punch 113 is inserted. Next, as shown in FIG. 8(B), the upper punch 112 is lowered to perform a half die cutting on the plate 100 in a state that the plate 100 remains to be connected. Subsequently, as shown in FIG. 8(C), the lower punch 113 is raised in the opposite direction and the portion on which the half die cutting is performed is moved back to the position where its upper face becomes the same height as the face of the remaining portion of the plate 100. Then, a product is manufactured by the lower punch 113 being further raised and punching.
According to the punching work by a press shown in FIG. 8, the forming of the burr 103 at the edge portion as shown in FIG. 7 is restricted since the portion is moved back in the opposite direction after the half die cutting is performed. However, the method described above is not suitable to a plate whose thickness is less than about 0.5 mm. In other words, in the case that the half die cutting is performed on a thin plate, it is important that the upper punch 112 is lowered to a depth or a position where a product can be punched out by the lower punch 113 being subsequently raised in the opposite direction. However, in the case that the thin plate with the thickness of not more than 0.3 mm is used, when the upper punch 112 is lowered to a depth where a product can be punched out by the lower punch 113 being raised in the opposite direction, the thickness of the portion remained to be connected becomes to be extremely thin. Therefore, the portion that is to be remained connected may be often punched out due to the physical property of material and thus the burr 103 may be formed.
On the other hand, when the thickness of the thin plate is set to be thicker such that the connected portion is not punched out, the burr 103 may be formed at the time when the lower punch 113 is raised to punch. Particularly, according to the press-working machine disclosed in Japanese Patent Laid-Open No. Hei 7-195130 and in Japanese Patent Laid-Open No. 2001-284116, the diameters of the holes 114 and 115 provided on the upper die 110 and the lower die 111 are set to be larger than those of the upper punch 112 and the lower punch 113. Therefore, the burr 103 is formed by the gap between the lower punch 113 and the hole 115 provided on the lower die 111.